Rectifier protective circuits



D. G. LITTLE ET AL. RECTIFIBR PROTECTIVE CIRCUITSv Nw. 1s, 1947.VV

Filed lay 20, 1943 2.0 40A Temperature? mvENToRs Dona/a GlLitzle 8f.WITNESSESz IMI WM CarlegY/f.' Hooper".

ATTORNE I Patented Nov. 18., 1947 RECTIFIER PROTECTIVE CIRCUITS DonaldG. Little, Baltimore. and Charles K.

y Hooper, Linthicum Heights, Md., assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation o! PennsylvaniaApplication May 20, 1943, Serial No. 487,748

Our invention relates to dry contact rectiilers, such as the well-knowncopper-oxide rectiers, and in particular relates to arrangements forprotecting such rectiiiers from overheating.

tiers are oi' the copper-oxide type, the seleniumtype, or oi other typesknown to the art.

It is also well known that rectiers ofthe contact type rapidly changetheir electrical characteristics with increase of temperature, and alsothat they are incapable oi withstanding voltages greater than a certaincritical amount impressed upon them in their non-conductive directionwithout suffering what may be termed a breakdown very similar to thedielectric breakdown of insulation. It is also found that the amount ofthis critical breakdown voltage decreases rather rapidly with increaseof temperature. AIn any actual service, the voltage at which therectiilers can 'operate under ywithout suffering such a breakdown is animportant factor in their rst cost; in other words, the rectiers have tobe designed with a suiiicient number of units in'series so that the linevoltage under which they are expected to operate will not cause theirbreakdown. The internal heat generated in the rectifiers in servicenaturally causes a rise in their temperature above that of theirsurroundings, and this lfact has to be taken into consideration indesigning a given rectifier, so that even with the temperature riseresulting from long continued operation at full load the impressed linevoltage is less than the rectifier breakdown voltage. Furthermore, iithe rectiflers overheat because of operating in excess of their ratedload capacity, or because of some rise in the ambient temperature toabnormal values, the resulting lowering of their breakdown voltage maycause the normal operating voltage to cause their breakdown andconsequent destruction.

Some means of giving an indication when the rectifier has exceeded thesafe operating temperature for the particular line voltage being used isaccordingly very desirable in practice. We have 4 Claims. (Cl. F15-363)discovered that the rapid increase of the reverse current through therectiiiers may be used as an expedient for giving warning when theirtemperature rises to a dangerous degree, and for op- Rectifiers of thedry-contact type generally pass 5 erating relays or other devices forcutting them a certain amount of current in the reverse direcout of theelectric circuit. tion, that is to say, in the direction in which theyOne object of our invention is accordingly to are nominallynon-conductive. This reverse curprovide dry contact rectiflers with aneillcient rent is relativelysmall compared with the -amount means forindicating their internal temperature. of current that they pass withoutoverheating in l0 Another object of our invention is to provide theirforward `cmnominally conductive direction. dry contact rectifiers with adevice which shall Investigation has shown that the reverse currentindicate when their internal temperature rises which will pass with agiven applied voltage in the above a safe value. reverse directionincreases quite rapidly as tem- Another object of our invention is toprovide .perature increases. This is true whether the rec- 15copper-oxide rectiiiers with an arrangement which shall indicate themagnitude of their resistance to current flowing in the reverse currentdirection.

Still another object of our invention is to Dro- ,vide copper-oxiderectiflers with means for removing-them from an electrical circuit whentheir operating temperature hasreached a predetermined value.

Other objects of our invention will become apparent from reading thefollowing description when taken in connection with the accompanyingdrawing, in which:

Figure 1 is a graphical plot showing the rela,- tion between the currentilowing through a contact rectifier in its non-conductive direction andthe temperature of the rectiiier; and

Fig. 2 is a schematic diagram of a rectier cir'- cult for making use ofthe principles of our in vention. f

Referring in detail to Fig. 1, temperature in degrees centigrade isindicated by the abscissae while the ordinates represent the reversecurrent, or the current which a certain impressed voltage will cause toflow through the rectifier in its nominally non-conductive direction. Itwill be evident from Fig. 1 that the reverse current rises slowly atfirst and at an increasingly rapid rate as the temperature rises. Forexample, the particular rectifier in question might be stable and ableto operate safely at a. certain line voltage at all temperatures belowAt temperatures wellA below 80, the amount of reverse current would bevery small, but in the neighborhood of 80, it is increasing rapidly, andthis abrupt increase of current may be use-d to indicate that therectiiler has attained a dangerous temperature, i. e., a temperatureabove 80 C. For example, a relay may be connected in circuit with therectifier which shall remain inoperative as long as the reverse currentthrough the rectifier is leas than that corresponding to 80 C. As soonas this temperature is exceeded, however, the relay may close a pair ofcontacts operating an alarm signal or even operating the tripping coilof acircuit interrupter which is in series with the rectifier.

Fig. 2 shows a-circuit in accordance with our invention which isespecially well adapted to utilize the rapid increase of reverse currentto operate an indicating device when the critical temfl'ers II and i aresuch that when line current is in the nominally conductive directionthrough the rectiflers I to 4, practically no current flows through thewindingl I2 because to do so this current would have to flow in thenominally nonconductive direction in the rectiner II. n the other hand,when reverse current tends to flow through the rectifiers I to l.current can readily flow through the winding I2 because the rectifierperature is reached. Fig. 2 shows one branch or limb of a rectifierwhich would conduct current in substantial amounts only in onedirection. In short, Fig. 2 shows a half-wave rectifier. In actualpractice, at least two such legs are usually connected by schemes wellknown in the rectifier art to conduct to a load both half-waves from thealternating-current source. For example, Grondahl Patent No. 1,640,335,assigned to the Union Switch & Signal Company, of Swissvale,Pennsylvania, illustrates a number of different connections forhalf-wave rectifiers to adapt them to conduct full-waves of directcurrent to a load when supplied by an alternating-current source.

The rectifier shown in Fig, 2 comprises a number of serially connectedunit rectifiers which may, for example, comprise copper plates oxidizedon one side, this side being thereafter coated with graphite andprovided with a contact plate of lead or other impressionable material.Such rectifiers are fully described in the above-mentioned Grondahlpatent. A plurality of such rectiilers, I, 2, 3, 4, 5 and 6 may beconnected in series with each other, the copper plate of one rectifiermaking contact with the contact plate of that adjacent to it. A leadwire 1. which may be connected to one side of an alternating-currentsource, is connected to, let us say, the copper plate of rectifier No.I, and a second lead wire 8 adapted to be connected to the other side ofthe same source is connected to the contact plate of rectifier No. 6.The structure so far described constitutes a conventional half-waverectifier. or one limb of' a full-wave rectifier of the dry-contact typeas known in the art.

Maintained in as good thermal contact as possible with, but separatedfrom, the rectifier B by an insulating layer 9, is a rectifier II, therectifier II being reversely positioned in comparison with the rectiersI. to 6, so that its contact plate is most closely adjacent to thecontact plate of the rectifier 6 and its copper plate most remotetherefrom. A connection is made from the contact plate of the rectifier5 to the copper plate of the rectifier II. Between the contact plate ofthe rectifier 6 and the contact plate of the rectifier II i. e.,connected so as to be shunted across the insulating layer 9, is thewinding I2 of a relay. The relay I2 is so adjusted that its contacts I3.I are normally open, but are closed by the amount of current which flowsthrough the rectifier 6 in the reverse direction when the normal linevoltage is impressed between the lines 'I, 8, and the temperature of therectifier is at the critical value at which dangerous overheatingbegins. The contacts I3, I 4 may be arranged to close the circuit of anydesired indicator, such as a signal or hell, or alternatively, as shownin Fig. 2, close the circuit between a. voltage source I5 and a trippingcoil I6 of a circuit breaker Il which is connected in series with line8.

It will be noted that the winding I2 is connected in series with therectier I I across the rectifier 6. The relative directions of therecti- II will then be transmitting this current in.its conductivedirection. Since the resistance to current flow in the last-mentioneddirection which is interposed by the winding I2 and the rectifier I I isarranged to be much smaller than the resistance to current flow inthereverse direction in the lrectifier' 8, practically all the reversecurrent which flows through the rectifiers I, 2, 3, 4 and 5 is shuntedaway from rectifier l and flows through the winding I2. The eect of theabovementioned connection of the rectifier II is accordingly to conductthe reverse current flowing through the rectifiers I, 2, 3, 4 and 5almost entirely through winding I2, but to prevent the flowthrough-winding I2 of the load current which flows through therectifiers I, 2, 3, 4, 5 and 6 in their nominally conductive direction.

If the temperature of' the rectifiers I to 6 rises beyond the safecritical value, the reverse current will be large enough to close therelay contacts I3, I4 and trip the circuit breaker II to cut therectifier out of circuit.

We have shown the rectifier I as positionedat the end of the rectifierseries, but the protective rectifier II, which may be termed anancillary rectifier, may be shunted across any one or more of therectifiers of a series such as I to 8. The ancillary rectifier II hasbeen described as a single unit, but for some purposes may comprise anumber of units in series. The ancillary rectifier II and the rectifiersit shunts may be of the same type as the rectifiers I to 6, or may be ofany different type having suitable rating to conduct the current andhaving other characteristics evident to those skilled in the art. Whilewe have described our protective device as applied to a single stack orseries of rectifier elements constituting a half-wave rectifier, theancillary rectifier may be shunted across any one rectifier in apluraity of half-wave rectiflers arranged to produce full-waverectification in any of the connections well known in the art for thatpurpose. In some cases, it may be desirable to provide an ancillaryrectifier and current-responsive element for each limb or half-waverectifier forming acomponent of such full-wave rectifier systems. Whilewe have shown the half-wave rectifier stack as comprising six unitrectifiers in series, it will be evident to those skilled in the artthat it may comprise any number from two up.

We have described a particular arrangement embodying the principles ofour invention, but these principles are of broader application in wayswhich will be evident to those skilled in the art.

We claim as our invention:

1. In combination with a plurality of rectifier elements connected inseries, a current path shunting a portion of said rectifier elements andcomprising an ancillary rectifier of opposite polarity in series with acurrent-responsive device. Y 2. Incombination with a plurality ofrectifier elements connected in series, a current path shunting one ormore but not all of said rectifier elements and comprising an ancillaryrectifier in series with a device, said current path being re- 5sponsive to the reverse current of said elements. 3. In combination witha plurality of rectifier elements connected in series, a` current pathshunting certain ones, but not all, of said rectier elements andcomprising an ancillary rectifier of reversed polarity in series with acurrentresponsive device controlling a circuit interrupter governingcurrent flow to said rectiiier elements. 4. In combination with aplurality of rectifier elements connected in series', a current pathshunting certain ones, but not all, of said rectier elements andcomprising an ancillary rectifier in series with a device, said currentpath being responsive to the reverse current of said eleterruptergoverning current ilow to said rectier elements.

x DONALD G. LITTLE.

CHARLES K. HOOPER.

REFERENCES crrEn The following references are of record in the 111e ofthis patent:

UNITED STATES PATENTS Y Number Name Date 1,802,014 Grondahl et al Apr.21, 1931 1,703,146 Holden Feb. 26, 1929 1,703,147 Holden Feb. 26, 1929ments, and said device controlling a cir'cuit in- 15

